Disclaimer: I only know that this works in the Boxer. I can try to help you adapt it to other layouts, but no guarantees because this is the only one I am familiar with.

Please note that this is not a tutorial for random background images; check s2howto for that. Their tutorial for random background images is where I got most of the code for this.

( code and directions, whee )

If anyone reading this actually knows what they're doing and has clarifications/corrections for these directions, I bow before your superior knowledge; please comment.